Conventionally, color television receivers employ a method of intentionally lowering reproduction of a direct-current component of a luminance signal so as to raise apparent contrast (Refer to, for example, Patent Document 1).
FIG. 7 shows a block diagram of one example of a conventional contrast adjustment circuit 1 that performs above-described signal processing, which includes an average luminance calculation unit 2 and a DC shift unit 3.
In the contrast adjustment circuit 1, the average luminance calculation unit 2 calculates the average level of a supplied luminance signal S1 over one field period or one frame period, and sends thus calculated average level to the DC shift unit 3 as an average level detection signal S2.
The DC shift unit 3 shifts a direct-current component of the luminance signal S1 depending on the average level of the luminance signal S1. That is, based on the average level detection signal S2, the DC shift unit 3 completely reproduces a direct-current component DC of the luminance signal S1 when its average level is “0” or all-black signal, and lowers reproduction ratio of a direct-current component as the average level becomes high.
FIG. 8 shows input-output characteristics of a luminance signal that undergoes above-described signal processing. In FIG. 8, f1 indicates input-output characteristics obtained by changing an input level of small area part of an image that hardly affects the average level when the average level of the luminance signal S1 is low, while f2 indicates input-output characteristics obtained by changing the input level of small area part of an image that hardly affects the average level when the average level of the luminance signal S1 is high.
As is apparent from FIG. 8, when the average level of the luminance signal S1 is high, the image somewhat becomes an all-black image (when an input level of f2 is between “0” to “a” in FIG. 8), while apparent contrast of the image seems to be raised as a whole.
[Patent Document 1]
Japanese Patent Application Laid-Open Publication No. 10-271409 (on page 3, FIG. 1).
However, in above-described conventional contrast adjustment circuit 1, since a direct-current component of the luminance signal S1 is shifted based on its average level to raise apparent contrast, an entire signal level of the luminance signal S1 is shifted, which undesirably lowers peak luminance (the maximum level of a luminance signal), as is apparent from FIG. 8.
Such phenomenon is not a big problem for a color television receiver using a CRT (Cathode-Ray Tube) as a display unit that can originally make the dynamic range of the highlight side luminance and that of the cut off side luminance large, but leads to a problem of deterioration of the highlight side luminance for a color television receiver using a liquid crystal panel or a plasma display as a display unit that cannot make the dynamic range of the highlight side luminance and that of the cut off side luminance large.